In the microelectronics industry, during the manufacture of an integrated circuit, surfaces that are typically scratch-free are polished for the purpose of planarizing the structure involved and/or removing unwanted material. The polishing involved is chemical mechanical polishing. For example, metals such as aluminum, copper, and tungsten are planarized. These metal surfaces are oxidized so that the polishing abrasive does not produce scratches. Moreover, there is typically a refractory metal liner beneath the aluminum, copper or tungsten providing good adhesion to the underlying insulator and good contact resistance to lower level metallizations. The liners can be niobium, tantalum and titanium alone or in combination with their nitrides, or any other refractory metal. In practice, it is often necessary to employ a multistep procedure in order to achieve the desired polishing results. For instance, a primary polish is employed for removing large or major amounts of the stock material, followed by a secondary polish for removing scratches caused by the primary polish and/or for removing a different underlying or liner material. Typically, these two polishing steps are carried out on separate polishing tables. However, there are often two or more polishing tables per polisher so that the entire procedure can be performed on one piece of machinery. Each aspect of each polishing step, such as the polishing pads, the mechanical rates of operation, and the polishing slurries are tailored for its particular function. Accordingly, the slurry that is used for the primary polishing step is often quite different in its chemical composition from that used in the second polishing step.
A problem often encountered results when one of the polishing slurries becomes contaminated such as from carry over of amounts of the other slurry, cleaning material or a biproduct of the polishing procedure. For instance, cross-contamination commonly occurs when minor amounts (e.g. a few drops) of the primary polishing slurry become entrained on the wafer or wafer carrier and is then deposited on the polishing platen for the second polishing step where it then becomes mixed with the second polishing slurry at the point of use of the second polishing slurry. In the event the primary slurry contains relatively high concentrations of chemicals or ions, even a few drops when mixed with the second slurry can result in diminishing the effectiveness of the second slurry. A frequent example of this occurs when an extremely acidic slurry is employed as the primary slurry and a relatively neutral slurry as the second slurry. In such a situation, even a few drops of the acidic slurry can cause the neutral slurry to become acidic, thereby diminishing its effectiveness. Previous attempts to overcome this problem include extensive rinsing of the wafer and carrier between polishing steps in deionized water and creating barriers between the polishing steps in order to prevent splashing of one slurry into the other. However, these prior solutions are not entirely satisfactory since the wafer must pass from one polishing platen to the other without drying and the two polishing tables are located close to each other. Accordingly, it would be desirable to overcome the contamination problem.